The progress of lasers in recent years has been remarkable and a high output and compact solid laser or semiconductor laser having a light emission region in the range from near infrared to infrared becomes easily available. These lasers are very useful as a light source for exposure in conducting the direct plate-making from digital data of a computer or the like.
A positive-working recording layer (hereinafter referred to as an image-forming layer) contains a binder resin soluble in an aqueous alkali solution and a dissolution inhibiting agent, which substantially decreases a solubility of the binder resin upon the interaction with the binder resin. Among them, the image-forming layer using an infrared laser for exposure preferably contains an infrared absorbing agent, for example, an infrared absorbing dye, which absorbs light and generates heat, in addition to the binder resin and the dissolution inhibiting agent.
Of the infrared absorbing agents, a cyanine dye is particularly preferably used as the infrared absorbing agent for the image-forming layer responding to an infrared laser, because it has also a function of the dissolution inhibiting agent.
However, in such a positive-working lithographic printing plate precursor for an infrared laser, there is a problem in that the difference between the dissolution resistance of the unexposed areas (image areas) to a developer and the solubility of the exposed areas (non-image areas) is yet insufficient under various using conditions, and an excessive development or an inferior development is liable to occur by the fluctuation of the using conditions.
Further, although an energy sufficient for an image-forming reaction can be obtained on the surface of the photosensitive lithographic printing plate precursor irradiated with a laser, diffusion of heat occurs. Particularly, in case of using a conventional aluminum support, there is a problem in that since the aluminum support has a good hest conductivity, the diffusion of heat to the support is severe and the energy cannot be sufficiently utilized for the image formation, resulting in low sensitivity.
In order to resolve such a problem, a method of providing an undercoat layer excellent in alkali solubility between the support and the image-forming layer has been proposed. In accordance with the method, after the imagewise exposure the undercoat layer excellent in alkali solubility is revealed in the exposed region by removal of the image-forming layer and an effect of smooth removal of an undesirable remaining film or the like with an alkali developer and an effect of efficiently restraining the diffusion of heat to the support due to the function of the undercoat layer as a heat insulating layer are achieved. As an image-forming material having a multi-layer structure, there is described a photosensitive image-forming material for an infrared laser comprising an upper layer having incorporated therein a conventional cyanine dye (refer to, for example, Patent Document 1).
However, from the standpoints of achieving high sensitivity regardless of variation on the light source of an exposure apparatus used for the image formation, obtaining a large allowance to fluctuation of concentration in the alkali developer (development latitude), and increasing storage stability of preventing, for example, the occurrence of aggregation of the infrared absorbing dye with the lapse of time, further improvements have been desired under the present situation.
Patent Document 1: JP-A-11-218914 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)